1. Field of the Invention
The present invention generally relates to surface acoustic wave elements, and more particularly, to a surface acoustic wave element that may be suitably used as a filter in a radio frequency section of mobile communication devices, in which such a filter is required to have advanced filter band-pass characteristics, particularly, higher breakdown power and an improved shape factor of the pass band. Further, the present invention is concerned with a duplexer using the above surface acoustic wave element.
2. Description of the Related Art
The surface acoustic wave element (SAW element), which is typically used as a SAW filter, has widely been applied to mobile communication devices such as cellular phones. Particularly, in recent years, there has been considerable activity in the development of replacement of a dielectric type antenna duplexer for separating transmitting and received signals from each other with the SAW element for the purpose of downsizing and reduction in the thickness of the mobile communication devices. It is essential to improve the breakdown power and the shape factor of the pass band of the SAW element in order to develop the antenna duplexer with the SAW technique. The reasons for such improvement are as follows. First, the antenna duplexer is placed just before the antenna, and amplified high power passes through the duplexer. If the SAW element does not have a sufficient breakdown power, it may be destroyed. Second, the recent mobile communication systems employ the transmit band and the receive band that are very close to each other. In order to prevent cross talk from developing between the transmit band and the receive band, the respective pass bands have sharp falling/rising edges. As described above, in order to achieve the antenna duplexer with the SAW technique, it is essential to improve the breakdown power and the shape factor of the pass bands of the SAW element.
There are two ways of improving the breakdown power of the SAW element. The first way is to improve the substance of the interdigital transducer that excites the SAW. The second way is to improve the design of the electrodes including the interdigital transducers. The first way is disclosed in, for example, Japanese Unexamined Patent Publication Nos. 5-90268 and 4-288718.
The second way is disclosed in, for example, Japanese Unexamined Patent Application No. 10-303698, in which the interdigital transducers respectively forming resonators of a ladder type SAW filter have slightly different electrode finger periods. Particularly, a unique arrangement for improvement in the breakdown power is proposed. According to the proposed arrangement, the electrode finger period of the series-arm resonator located at the first stage viewed from the input side is smaller than that of another series-arm resonator, while the electrode finger period of the parallel-arm resonator located at the first stage viewed from the input side is larger than that of another parallel-arm resonator. The above-mentioned publication reports that the above-mentioned settings of the electrode fingers enable the resonance frequency at which the breakdown power becomes the weakest to shift to the outside of the pass band. More specifically, according to the publication, the resonance frequency of the series-arm resonator at the first stage is set higher by reducing the electrode finger period thereof, and the resonance frequency of the parallel-arm resonator at the first stage is set lower by enlarging the electrode finger period thereof. The above-mentioned arrangement disclosed in the publication greatly improves the breakdown power of the SAW filter and enables practical antenna duplexers that have pass bands in the range of 800 MHz to 1 GHz.
The recent mobile communication systems employ a pass band in the range of 1.8 GHz to 2 GHz, which is higher than the conventional pass band frequencies. Such higher pass band frequencies require further improved pass band shape. That is, the SAW filters used for the higher-frequency band would not be achieved unless both the breakdown power and pass-band shape are simultaneously improved. Shifting the pass band towards higher frequencies needs improvement in the breakdown power of the SAW filter. It is to be noted that the frequencies of the pass band are inversely proportional to the electrode finger period of the interdigital transducer. That is, the higher the pass band frequency, the shorter the finger period. As a result, each finger becomes narrower, which degrades the breakdown power characteristic.
Conventionally, it is very difficult to simultaneously improve both the breakdown power and pass band shape of the ladder-type SAW filter. This is because the breakdown power of the ladder-type SAW filter depends on the frequency. It is known that the frequency at which the breakdown power becomes the weakest is located in between the pass band and the rejection range, as disclosed in FIG. 3 of Japanese Unexamined Patent Publication No. 10-303698. If the shape of the filter pass band is improved so as to have sharper rising/falling edges, the pass band and the rejection range will become closer to each other, so that the frequency for the weakest breakdown power becomes very close to the pass band. In this case, the breakdown power is greatly lower than that of the SAW filter that does not have a good band shape.